The present invention relates, in general, to a process for depositing III-V semiconductor compounds as epitaxial films onto semiconductor substrates which are crystallographically compatible with said films. In a more particular aspect, this invention relates to a process for effecting a continuous, insitu, hydrogen chloride etch during the vapor phase epitaxial deposition of III-V mixed compounds onto semiconductor substrates. The resulting epitaxial structures find particular utility in the fabrication of double heterostructure lasers, light-emitting diodes and field effect transistors. III-V semiconductor compounds, as is well understood by those skilled in semiconductor technology, are those which include elements from group III and group V of the Periodic Table of Elements. The components can be binary, ternary or quaternary mixtures of any of those elements.
The recent interest in the development of epitaxial structures for use in a variety of semiconductor applications has spawned a considerable research effort in an attempt to grow high quality epitaxial films on semiconductor substrates. Heretofore, it has been extremely difficult to obtain epitaxial layers of III-V compounds that exhibit good morphology and possess relatively few impurities.
The need for high quality epitaxial structures is most important in that area of optical communications technology which forms an integral part of present day tactical weapons systems. Lasers and photodetectors are two of the essential components in optical communication systems and the III-V epitaxial film coated substrates are the basic structures used in the fabrication of these components. The use of high quality III-V epitaxial structures of good morphology with defect-free surfaces dramatically improves the efficiencies and life time of the lasers and photodetectors, thus providing a much more efficient and reliable tactical communications system.
The III-V epitaxial structures used for semiconductor applications, such as those employed in the fabrication of light-emitting diodes, photo-detectors and double-heterostructure lasers, generally include the arsenides, antimonides, phosphides and nitrides of aluminum, boron, gallium and indium, as well as ternary and quaternary mixtures thereof. These compounds, in general, are grown as crystalline films on a semiconductor substrate by either vapor phase or liquid phase epitaxial techniques.
In vapor phase epitaxy, a number of specific processes have been suggested for effecting the deposition of a III-V compound film. These processes usually include the steps of reacting two gaseous mixtures within an enclosed reaction chamber to provide a III-V compound. The two gaseous mixtures generally utilized in vapor phase epitaxy comprise as one of them, a first gaseous mixture formed by contacting a Group III element with hydrogen halide; while the other, or second gaseous mixture, is formed by mixing hydrogen, as a carrier gas, with a Group V element in gaseous form. The III-V compound resulting from the interreaction of the two gaseous mixtures is then deposited as an epitaxial film onto a suitable semiconductor substrate. The semiconductor substrate may be similar or different than the material used to form the epitaxial films and includes III-V compounds, II-VI compounds, as well as silicon and germanium.
One of the better known methods for producing III-V compounds is referred to as the vapor phase epitaxial hydride technique (VPE-Hydride). The specific details of this technique are set forth in a review paper by G. H. Olsen and T. J. Zamerowski, "Crystal Growth and Properties of Binary, Ternary and Quaternary (In, Ga) (As,P) Alloys grown by the Hydride Vapor Phase Epitaxy Technique", B. R. Pamplin (ed): Progress in Crystal Growth and Characterization, Vol II, Pergamon Press Ltd., London (1979), pp 309-375.
Unfortunately, the growth of III-V compounds, especially in the preparation of InP and the quaternary alloys (In,Ga)(As,P), is difficult and the resulting crystalline films lack good morphological characteristics and often possess a high number of impurities. The defects produced during the growth of epitaxial films originate from a number of sources, e.g., dislocation on the substrate, inappropriate epitaxial growth conditions, and the presence of foreign matter or impurities during the growth process. Since these particular compounds are of great importance in the fabrication of double heterostructure lasers and light-emitting diodes, a considerable research effort is currently being devoted to the specific preparation of InP and InGaAsP quaternary compounds which possess good morphological characteristics.
As a result of this research effort, it was found that the application of a continuous, in-situ, hydrogen chloride etch during the actual growth of the epitaxial layer promoted the formation of an epitaxial crystalline film with considerably fewer defects than that achieved heretofore by prior art methods. The hydrogen chloride etching material is added to the mixing zone of a conventional quartz reaction tube in addition to the hydrogen chloride which is conventionally added to the source zone of a typical three zone reactor, such as that used in the VPE hydride technique. The hydrogen chloride etch added to the mixing zone then flows into the deposition zone and provides the opportunity to remove impurities and unstable species that give rise to defect laden epitaxial films. The role of the HCl etch in the mixing zone, as well as in the deposition zone, appears to be the factor responsible for the lower carrier concentrations and higher mobilities obtained with the epitaxial InP layers grown by this modified VPE-hydride method.
In carrying out the method of this invention, the growth rates and morphological character of the epitaxial layers were studied as functions of temperature, amount of hydrogen chloride in the mixing zone, and flow rates of phosphine and source HCl (this being the HCl in the source zone which reacts with the elemental indium source to form InCl). Epitaxial layers with good morphology were observed when hydrogen chloride was added to the mixing zone at relatively low flow rates.
Heretofore, the in situ, non-continuous etching of substrates has been studied extensively in the preparation of GaAs and InP by the VPE-hydride technique. This preliminary non-continuous etching ensured a clean and damage-free substrate prior to epitaxial growth. The source HCl used in the the non-continuous etching processes was generated from the reaction of a group V trichloride with hydrogen. Unfortunately, however, the epitaxial layers produced heretofore were not sufficiently free from defects to the degree necessary to insure their use in heterojunction laser and light emitting diode applications.
Accordingly, the primary object of this invention is to provide a novel method for forming Group III-V epitaxial films that are relatively free from defects and possess good morphological characteristics.
Another object of this invention is to provide a novel method for forming Group III-V epitaxial layers on suitable semiconductor substrates.
Still another object of this invention is to provide a novel method for depositing a Group III-V epitaxial film by vapor phase epitaxy in which a continuous hydrogen chloride etch is maintained during the actual deposition and formation of the epitaxial film.
The above and still other objects and advantages of the present invention will become more readily apparent upon consideration of the following detailed description thereof when taken in conjunction with the following drawing.